Quantifying the drivers of longleaf pine regeneration

Background/Question/Methods  

Longleaf pine (Pinus palustris Mill.) is a keystone species that modifies its environment through fire facilitation which provides a unique fire dependent habitat for many rare and endangered  endemic species.  However, many aspects longleaf pine regeneration are not well-understood.  This project is intended to provide a better understanding of regeneration within longleaf pine savannas by: investigating the relationship between regeneration patterns and the amount of light reaching the forest floor through the overhead canopy; quantifying the effect of ground cover composition, land use, and disturbance history on regeneration; and examining the role of intraspecific competition in the growth rates of seedlings and saplings.  We surveyed 648 randomly selected quadrats within long-term monitoring plots at the Joseph W. Jones Ecological Research Center.  All longleaf pine seedlings (<2m height) and saplings (>2m height and <10cm diameter at breast height) were measured.  Groundcover percentages were estimated and photosynthetically active radiation (PAR) was measured at ground level at each transect and above the canopy at nearby eddy covariance tower locations.  Hemispherical photos were taken at a subset of quadrats and gap fraction was estimated. We hypothesized that light would be the primary driver of regeneration dynamics.

Results/Conclusions

The results of a zero-inflated Poisson model indicated that the presence of seedlings and saplings was significantly related to land use history, overstory PAR, gap fraction, and overstory density.  Given that regeneration was present, the density of seedlings was significantly controlled by both under- and overstory PAR and overstory density.  Seedling abundance was not significantly related to ground cover or recent disturbance history.  The results of a general linear model revealed that intraspecific competition among longleaf pine saplings as measured by the ratio of height to diameter (H/D) was positively correlated with the number of seedlings and saplings within each quadrat.  The ratio of H/D for saplings growing alone within a quadrat was approximately one and increased to ~1.5 as the number of seedling and saplings within the quadrat increased. The initiation of seedling height growth was also positively correlated with gap fraction.